Known techniques for reducing an amount of noise of a moving image include a cyclic noise reduction technique. This technique performs motion compensation between an input frame image and a processed previous frame image, and computes reliability of the motion compensation related to each pixel based on differences from peripheral pixels. The input frame image (the current frame) and the previous frame image are blended and synthesized in a blending synthetic ratio (a mixing ratio) derived from a reliability, an amount of motion, and an estimated amount of noise (see Japanese Patent No. 4321626 and Japanese Patent Application Laid-open No. 2010-147986).
In recent years, a motion detection for each pixel by recording a moving image at high image quality and high frame rate increases the circuit size and the computation time. On the other hand, a motion compensation based on a motion detection with discrete points reduces the circuit size and the computation time. In this case, an error occurs in the motion detection. In a block matching method and similar method that allow a high-speed motion detection, an error occurs in a motion detection due to influences of, for example, lens distortion and rolling shutter. Additionally, an error occurs in a motion detection due to an influence of noise. Therefore, the known technique that reduces the amount of noise increases the synthetic ratio of the previous frame image as the reliability of the motion compensation becomes higher, and reduces the synthetic ratio of the previous frame image as the reliability of the motion compensation becomes lower, so as to prevent occurrence of image lag and artifact during the blend synthesis. It should be noted that the reliability of the motion compensation can be computed from an evaluation value for dissimilarity, for example, the sum of absolute difference (SAD) between the images processed by the motion compensation. The reliability increases as the sum of absolute difference becomes smaller, and decreases as the sum of absolute difference becomes larger.